Yes they are very different. Many see VSK/AC32 as a match-racing simulator where as SailSimulator is a sailing simulator.

VSK does not have the ability to adjust the boat setup so all racing is essentially "one-design". VSK has a rules engine, umpire and in-game penalties. VSK is almost dedicated to online play - the novelty quickly wears off if you want to play offline as the simulation boats are pretty dumb and are predictable.

It really depends on what your looking for - if your looking to simulate sailing a yacht then SailSim wins hands down... if you looking for online multiplayer tactics and race simulation then VSK is currently the better. Having said that if you were to compare the future potential then your best investment is Sail Simulator because there is active product development and a potential to add good racing and regatta simulation features (such as in-game rules). On the VSK side there is no active development and the future is unclear since VSK developer Nadeo was acquired by Ubisoft - and they did not buy Nadeo for VSK I can tell you that

But on the flip side if your looking for online competition today you will find VSK has a large online community and many fansites with organised worldwide regattas and events. I look forward to see how SailSim evolves in the area.